A very interesting article explaining the rules of composition --
What is a photograph? It is a story. What is a story? It is is a series of sentences connected to each other. The same is true about photography. To create a photograph, it is not enough just to take an image of something. The first impression from a photograph is determined by the composition balance of an image.
To increase the expressiveness of your digital pictures, apply the picture composition rules while taking the photos or modeling their edges.
Read more here..
http://www.colorpilot.com/comp_rules.html